Inorg. Chem. 1987, 26, 2332-2334
2332
-
-
state of the molecule. The excited states of both the ligand n a* transition and the M(LH2),X2 M(d) L a* transitions are derived from the lowest unoccupied a* orbital of the ligand. Therefore, the bonding interactions and percent character composition for the two excited states are qualitatively very similar. This interpretation of the spectrum allows several observations to be explained. The initial weak band at 704 nm is a M(d) L a* transition rather than a d d transition as previously advanced but exhibits a weak coupling to a vibrational mode resulting in a low intensity. The loss of this band in the solution spectrum (Figure 3a) can be explained with the assumption, in analogy to the free ligand, that this band gains most of its intensity by coupling to a lattice mode that would not exist in the solution spectrum. In addition, it has been reported2Icthat in the solid-state KBr spectra for the M(N,N’-Me2dto)zBr2complexes (M = Ni, Pd, Pt) the relative intensities and energetic spacings of the absorption bands in the visible region were scarcely influenced by the central metal. This observation can be explained quite readily according to our interpretation of the spectral data, which states that the relative intensities and energetic spacings of these bands would not be dependent on the central metal but rather on the energy of the coupled vibrational frequencies and on the intensity pattern of their associated Franck-Condon progressions. Summary In agreement with the previous spectral assignment of dithiooxamides,20 the UV-vis spectrum for dithiooxamide has been shown to consist of two bands. The first band is a weak broad n a* transition occurring in the visible region and is assigned ]A, (13a, 4a,). The second band is an intense broad as ‘A, a a* transition occurring in the UV region and is assigned as ‘A, (3b, 4a,). Symmetrical N,N’-dialkylation of the ‘B, dto moiety has been shown to cause a blue shift and an increase
-
-
+ -
+ -
-
in the molar absorptivity value for both of the transitions. A red shift of the transitions was observed for N,N’-bis(trimethy1sily1)dithiooxamide. The n a* transition of the dithiooxamides reported here exhibits heretofore unreported vibrational fine structure, which clearly resolves upon temperature reduction. A preliminary analysis of the spectral data of N,N’-Bz1,dto indicates that at least three vibrational frequencies are contributing to the observed spectrum. The two lower energy frequencies of ca. 52 and ca. 180 cm-I are tentatively assigned as crystal lattice modes, while the major vibrational progression with a frequency of ca. 1150 cm-’ appears to involve the C-C stretch. The spectra for Ni(N,N’-B~l~dto)~Br,, which is representative of the M(LH2)2X, complexes, have been shown to consist of a series of bands in the visible region of varying intensity and an intense broad band in the UV region. In contrast to the previous assignment?lc the UV band is assigned as a L a L a* transition. In partial agreement with the previous assignments,21a,cthe bands in the visible region for the M(LH2)2X2complexes are assigned as M(d) L a* transitions. However, the weak low-energy bands d transitions as previously advanced but are M(d) are not d a* transitions. The multiplicity of bands in the visible region is caused by the coupling of one or more vibrational modes to L a* transitions. The major vibrational proallowed M(d) gression resolves upon temperature reduction and appears to be qualitatively very similar to the major vibrational progression observed for the n a* transition of the free ligand.
-
-
-
-
--
-
-
-
Acknowledgment is made to the 3M Corp. and to the Research Corp. for partial financial support of this work. M.R.G. is grateful to The Ohio State University Graduate School for his support as a Presidential Fellow. We thank Roger H. Cayton for assistance in obtaining the low-temperature spectra.
Notes Contribution from the Department of Chemistry, The University of North Carolina, Chapel Hill, North Carolina 27514
Intervalence Transfer in the Dimer [ (NH3)5Ru11( 4,4’-bpy)Ru111(NH3),$’ Joseph T. Hupp and Thomas J. Meyer* Received June IO, I986
In the study of mixed-valence compounds, ligand-bridged dimers based on the [(NHJ)5R~111/11] couple have played a central role.] Notable examples include the Creutz and Taube ion,, [(NH3)5Ru(pz)Ru(NH3)5]s+ (pz = pyrazine), where the question of localization vs. delocalization is a source of continuing debate,’-j and the analogous 4,4’-bipyridine-bridged dimer, [ (NH3)5Ru11( 4 , 4 ’ - b p y ) R ~ ~ ~ ~ ( N Hwhere , ) ~ ] ~the + , valences appear to be trapped and the properties are well-defined in terms of available t h e ~ r y . ~ We have reinvestigated a particular property of the latter ion, the solvent dependence of the energy of its intervalence transfer (IT) or metal-metal charge-transfer (MMCT) absorption band. On the basis of the results of our study and in light of recent developments in the area, we conclude that (1) there is a con(1) See, for example: (a) Creutz, C . Prog. Inorg. Chem. 1983, 30, 1. (b) Taube, H. Ann. N.Y. Acad. Sci. 1978, 313,481. (2) Creutz, C.; Taube, H. J . A m . Chem. SOC.1969, 91, 3988; 1973, 95, 1086. (3) Fuerholz, U.; Biirgi, H. B.; Wagner, F. E.; Stebler, A,; Ammeter, J. H.; Krausz, E.; Clark, R. J. H.; Stead, M.; Ludi, A. J . A m . Chem. SOC. 1984. 106. 121. (4) (a) Tom, G. M.; Creutz, C.; Taube, H. J. A m . Chem. SOC.1974, 96, 7828. (b) Creutz, C. Inorg. Chem. 1978, 17, 3723.
0020-1669/87/1326-2332$01.50/0
siderable contribution to the apparent IT band energy arising from the existence of multiple IT transitions, (2) application of dielectric continuum theory to the solvent depencence of the IT band energy is not quantitatively successful, and (3) solvent effects whose origins arise at the molecular level may play a role in dictating the observed absorption band energies. According to Hush,S the band energy (Eop)for intervalence transfer in a chemically symmetrical mixed-valence dimer where electronic coupling is energetically negligible is given by Eo, = Xi +
xs
where xi and are the intramolecular (inner coordination sphere) and solvent reorganization or trapping energies, respectively. From dielectric continuum theory, xs is given by5
In eq 2, e is the unit electronic charge, r is the molecular radius of the redox sites, d is the separation distance between trapping sites, and Dopand D, are the optical and static dielectric constants of the solvent. The result in eq 2 is based on the assumption that the redox sites can be treated as nonpenetrating spheres. The effect of solvent has also been treated on the basis of the electronic redistribution that occurs within an ellipse enclosing the ion.6 For the case of interest here, the two-sphere model would appear to ( 5 ) Hush, N. S . Prog. Inorg. Chem. 1967, 8, 391.
(6) (a) Brunschwig, B. S.;Ehrenson, S.; Sutin, N. J . Phys. Chem. 1986, 90, 3 6 5 7 . (b) Cannon, R. D. Chem. Phys. Lett. 1977, 49, 299. (c) Kharkats, Yu I. Electrokhimya 1976, I S , 1251. (d) German, E. D. Chem. Phys. Left. 1979, 64, 305.
0 1987 American Chemical Society
Inorganic Chemistry, Vol. 26, No. 14, 1987 2333
Notes
-
orbit) states Edv(2) and Ed.( 1) aslo 40
9
EIT(2)(dn2
I
-
EIT(l)(d~3 dr;)
9.51
EIT(3)(drI
--*
dr3’) = E I T ( l ) + Ed,(l)
(5)
d~3’)= EIT(l) + Edr(2)
(6)
In fact, multiple IT transitions are observed for mixed-valence osmium dimersI0J4where the spin-orbit coupling constant is large (Aos 3000 cm-I). Excited-state IT transitions (drFe dr*Fe) have also been reported in cyanoiron mixed-valence solids.I2 For Ru, where the spin-orbit coupling is lower (ARu 1100 cm-I), the three IT bands are, in general, not resolvable. However, of the three IT components the two arising from a linear combination of d,, and dyr (defining the z axis as lying along the Ru(4,4’bpy)Ru axis) should be the most allowed, because of extensive mixing with ~*(4,4’-bpy)levels, and these should provide the majority of the intensity to the IT manifold. Stabilization of d,, and d,, by mixing with ~*(4,4’-bpy)also means that the transitions d,, d,,’ and d,, d,?’ will occur on the high-energy side of the IT manifold. The analysis outlined above provides a qualitative basis for understanding the large intercept obtained in the solvent dependence plot. Equation 1 is only applicable to the lowest of the three IT transitions. The actual IT band energy will include contributions from all three components at energies EIT(l ) , EIT(1) + E%(l), and EIT(1) &(2) with the intensity distribution skewed toward the higher energy components. E,, is related to EIT(1) EIT(l) A, where A is the energy by the relationship E,, difference between the absorption band maximum arising from the higher energy components and EIT(1) (eq 7).
-
0.5
0.4 l/%1/&
Figure 1. Intervalence transfer band energies vs. (l/DOp- l/Ds): (0) this work; ( 0 )ref 4. Key to solvents: BN = benzonitrile, N B = nitrobenzene, D M S O = dimethyl sulfoxide, DMF = dimethylformamide, D M A = dimethylacetamide, PC = propylene carbonate, F A = formamide, N M = nitromethane.
Scheme I
(4)
-
-
-
-
+
-
+
Eo, = Xi + x o + A
(7)
- -
In the related Os(II1)-Os(II1) dimer [(NH3),0s(4,4/-bpy)Os(NH3)#’+,inter d, transitions are observed at Ed (1) 4800 and Ed,(2) 5600 cm-l.13 Since ARu 1100 cm-? and Aos 3000 cm-I, Edr(l) and & ( 2 ) for the Ru dimer are predicted to occur at approximately 1800 and 2100 cm-].I4 Taking an average value of 1900 cm-’ and assuming that the intensity of the IT hv manifold is dominated by the d,,,d,,-based transitions gives A [ (NH~)~RU”(~,~’-~P~)RU”’(NH~)~]~+ 1900 cm-I. Since the intercept should equal the sum of xi and [(NH3)5Ru111(4,4/-bpy)Ru11(NH3)5]5+ (3) A, a A value of this magnitude would help to account for the large intercept in the solvent dependence experiment. From the plot, a linear correlation exists although the point for From eq 2 and dielectric continuum theory it is possible to D 2 0 is three standard deviations (u = 170 cm-I) above the best-fit calculate the slope of the E,, vs. (l/D,, - l/Ds) plot in Figure line for the remaining solvent. Omitting D208gives an intercept 1. Using r = 3.5 h; and d = 11.3 h; gives 6E0,/6(1/D,, - l/Ds) of 4820 cm-I and slope of 7810 cm-I, which differ substantially = 22 500 cm-I, which is only 35% of the experimental value. The from those reported earlier (intercept = 2500 cm-’; slope = 13 500 disagreement is particularly striking in view of the nearly quancm-I), which were based on fewer solvents and included D20.4b titative agreement with dielectric continuum theory found for the From eq 1 and 2 the intercept of the plot is predicted to equal dimer [(bpy)2C1Ru111(4,4’-bpy)Ru11C1( bpy),] 3+.15 The lower than xi. From bond length data for [(NH3)sRu(pz)]2+ and expected sensitivity to solvent for the pentaamine-based dimer has [(NH3)sRu(pz)]3+(pz is p y r a ~ i n e )Creutz ,~ has calculated xi = also been observed by treating the earlier, somewhat limited solvent 1400 cm-l$b giving a calculated intercept that is far less than that dependence data by using the ellipsoidal cavity model.6a It was estimated by using the data presented here. In retrospect, the suggested there that the origin of the effect may lie in specific discrepancy is expected when a more detailed analysis is applied interactions with coordinated NH,, which would tend to restrict to the IT “bandn.6a,10,11In symmetries lower than Oh,including the motion of nearest neighbor solvent molecules, which would spin-orbit coupling, the three d, orbitals, which are initially t,, be manifested as a local saturation of the solvent dielectric. and degenerate in Oh symmetry, are split into d,,, d,,, and dr3. At the very least, the discrepancy shows that dielectric conAs shown in Scheme I the lifting of the degeneracy in the d, levels tinuum theory does not provide a satisfactory quantitative basis leads to three possible IT transitions. The energies of the corfor understanding the role of solvent in the IT transition for the responding IT absorption bands are predicted to be separated decaammine dimer. There are some additional implications of approximately by the energies of the Ru”’-based inter d,‘ (spinthe result that are notable. (1) In contrast to the optical experiment, the solvent contribution to thermal self-exchange for (7) E,, values were obtained from near-IR spectra by using a Cary 17 reactions like [ (NH,),Ru(py)] ,+I2+ appears to be well described spectrometer. The preparation of [(NH3),Ru(4,4’-bpy)Ru(NH,)S!by dielectric continuum theory.16 It is difficult to imagine that (PF,), is described in ref 4. The mixed-valence form was obtained in be as appropriate as the ellipsoidal cavity model and the results have the advantage of being physically more transparent for the two-sphere model. In Figure 1 is shown a plot of the IT transition energy (E,,) vs. (]/Do, - l/O,)’ for the transition +
(8) (9) (10) . . (1 1)
situ by using Br, vapor as oxidant or by preparing the fully oxidized PF,-,Br- mixed salt in solid form and combining it with an equimolar quantity of the fully reduced complex. Including D 2 0 : slope = 9010 cm-’; intercept = 4300 cm-I. Gress, M. E.; Creutz, C.; Quicksall, C. 0.Inorg. Chem. 1981, 20, 1522. Kober, E. M.; Goldsby, K. A.; Narayana, D. N. S.; Meyer, T. J. J. Am. Chem. SOC.1985, 105, 4303. (a) Goldsby, K. A. Ph.D. Thesis, The University of North Carolina, 1983. (b) Kober, E. M. Ph.D. Thesis, The University of North Carolina, 1982.
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-
(12) Robin, M. B.; Day, P. Adu. Inorg. Chem. Radiochem. 1967, 10, 247. (13) Magnuson, R. H.; Lay, P. A.; Taube, H. J . Am. Chem. SOC.1983,105, 2507. (14) A more exact analysis would calculate EdT(l)and Ed (2) as the sum of distinct contributions from spin-orbit coupling and Ggand field asymmetry. (15) Powers, M. J.; Meyer, T. J. J . Am. Chem. Soc. 1980, 102, 1289. (16) Brown, G. M.; Sutin, N. J . Am. Chem. SOC.1979, 102, 883.
Inorg. Chem. 1987, 26, 2334-2337
2334
the theory would be adequate for one problem and not the other, and perhaps additional ingredients exist in the self-exchange kinetics that lead to a fortuitous agreement or perhaps a simple relationship between optical and thermal electron transfer does not exist for ammine complexes. (2) One criterion that has been cited in support of a delocalized description for the dimer [(NH3)sRu(pz)Ru(NH3)5J5+ is the absence of a significant solvent dependence.’ However, given the relatively slight dependence of E,, on solvent in the 4,4’-bpy dimer, noting that in the related, 3+ the subvalence-trapped system [(bpy)2C1Ru(L)RuCl(bpy)2] stitution of L = 4,4’-bpy by L = pyrazine decreases 6E0,/6( 1/Do, - 110,) by twofold, the solvent dependence criterion for delocalization may not be fully valid. (3) From the results obtained in an earlier experiment in which the solvent dependence of the metal-to-ligand charge-transfer band energy in [(NH,),Ru”(py)] 2+ was observed, it was concluded that solvent effects were dominated by the way in which specific solvent-ammine H-bonding interactions influenced the energy separation (AE)between the excited and ground states.!’ In that experiment E,, could be correlated with an empirical solvent basicity parameter, the so-called donor number, which is derived from the heat of solvation of SbCl5.Is In the decaammine dimer where AE = 0 (except for the small contribution from multiple IT transitions (A)), the specific solvent effect must appear in the solvent trapping term xS.l9 We find that multiple parameter fits of the type (Eop-f(donor number)) vs. (l/D,, - l/Ds) do give slightly improved correlations. In a more appropriately designed theory, specific solvent moleculemetal complex interactions may need to be included explicitly.20 Finally, although xs for D 2 0 falls within 3u of the best-fit line, the positive deviation is probably significant since it appears to be a general phenomenon for mixed-valence complexes in water, independent of the ligand coordination environment.21 We have speculated elsewhere2’ that one possible origin is in a contribution to solvent trapping from relatively high frequency solvent librations that have been observed in the Raman spectrum of water.22 Acknowledgments are made to the U S . Army Research Office under Grant No. DAAG29-85-K-0121 for support of this research. Registry No. BN, 100-47-0; NB, 98-95-3; M e 2 S 0 , 67-68-5; DMF, 68-12-2; DMA, 127-19-5; PC, 108-32-7; FA, 75-12-7; NM, 75-52-5;
[(NH~)~RU~’(~,~’-~~~)RU”’(NH~)~]’+, 54065-65-5. Curtis, J. C.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1983, 22, 224. Gutmann, V. Electrochim. Acta 1976, 21, 661. Evidence for such effects exists in related systems: Curtis, J. C., private communication. An attempt to account for such effects on a phenomenological basis has appeared: Hupp, J. T.; Weaver, M. J. J . Phys. Chem. 1985,89, 1601. Hupp, J. T.; Meyer, T. J. J . Phys. Chem. 1987, 91, 1001. Eisenberg, D.;Kauzmann, W. The Structure and Properties of Water; Oxford University Press: Oxford, England, 1969; pp 242-245.
Contribution from the Department of Chemistry, University of California, La Jolla, California 92093
Temperature Dependence of the Doublet Lifetime in Chromium(II1) Compounds
Gail E. Rojas and Douglas Magde* Received December 16, I986
The role of the lowest excited doublet state in chromium(II1) photochemistry has been investigated diligently for the past quarter of a century.‘I2 Among numerous reviews, that by Kemp3 is particularly pertinent to the issues we address here. The lowest doublet, unlike excited quartet states, frequently emits detectable (1) Chen, S.-N.;Porter, G. B. Chem. Phys. Lerr. 1970, 6, 41. (2) Kane-Maguire, N. A. P.; Langford, C. H. J . A m . Chem. Soc. 1972, 94, 2125. (3) Kemp, T. J. Prog. React. Kiner. 1980, IO, 301
0020-166918711326-2334$01 SO10
phosphorescence even in room-temperature aqueous solutions, which facilitates the measurement of photophysical properties and encourages experimental investigations. With the introduction of lasers and improvements in electronic detection, it is now possible to characterize a wide variety of different Cr(II1) compounds including, in particular, those with short lifetimes and very low luminescence yields. Ligand substitution reactions may occur directly from the doublet state in some, many, or all compounds, but such reactions occur in competition with other decay mechanisms. The very basic question that we address here is, Which decay process is “lifetime-controlling” under what circumstances? Assuming conditions such that there are no diffusive bimolecular quenching processes, it is common to distinguish four decay processes: nonradiative relaxation to the ground state k”,; phosphorescence krad;chemical reaction k,,,; and reverse intersystem crossing RISC to an excited quartet k,,,,. Within this model, the doublet lifetime would be expressed as /rphos
=
krxn
+
krisc
+ knr + krad
The radiative rate is extremely small and nearly temperature i n d e ~ e n d e n t . ~The nonradiative rate usually refers only to a weak-coupling process that can be characterized at low temperatures. It also is fairly small, and its temperature dependence is insufficient to account for the large overall decay rates at room temperature.j The remaining two terms are expected to be more strongly temperature dependent. Either one or the other may be invoked to explain the fact that in room-temperature solutions the doublet lifetime is usually much shorter and displays more variation with temperature than it does under cryogenic conditions. A minor detail is that RISC need not be fully irreversible; there could be some probability of return to the doublet. That does not change the logic of the model; it simply requires multiplying krisc by a branching ratio. The above scheme is logically incomplete. One should entertain the possibility of additional nonradiative pathways that dissipate electronic energy as heat but do not lead to net chemical reaction. These might involve substantial nuclear motions and be inhibited in glassy or crystalline matrices at low temperatures but assume dominance in fluid solution at ambient temperatures. Since the pioneering work of Targos and Forster5 showing that the temperature dependence of nonradiative decay can be quite complicated, even at low temperatures, there have appeared several additional studies on the same subject, which have received critical re vie^.^ The relevance to room-temperature processes remains obscure. The common practice6-” has been to persist with the four mechanism model summarized above and argue that “if it is not reverse intersystem crossing, then it must be chemical reaction that dominates.” Forster’ certainly recognizes that the short phrase “chemical reaction” needs further elaboration; presumably others do as well. He considers the possible role of geminate recombination following a “primary” dissociative reactive step. Endicott and co-workers12propose a mechanism that they consider to be distinct from both nonradiative decay and chemical reaction, but which, in a way, accommodates both. They suggest a curve crossing that populates a “reactive ground-state intermediate” from which a branching is possible to form either photoproducts or the original ground state. They have in mind an associative intermediate or transition state. Previous studies have attempted to identify processes occurring from the doublet by using a variety3 of different kinds of mea(4) Bjerrum, J.; Adamson, A. W.; Bostrop, 0. Acta Chem. Scand. 1956, 10, 329. ( 5 ) Targos, W.; Forster, L. S.J . Chem. Phys. 1966, 44, 4342. (6) Shipley, N. J.; Linck, R. G. J . Phys. Chem. 1980, 84, 2490. (7) Forster, L. S.; Castelli F. J . Phys. Chem. 1980, 84, 2492. (8) Adamson, A. W.; Gutierrez, A. R. J . Phys. Chem. 1980, 84, 2492. (9) Walters, R. T.; Adamson, A. W. Acta Chem. Scand. Ser. A 1979, A33, 53. ( I O ) Gutierrez, A. R.; Adamson, A. W. J . Phys. Chem. 1978, 82, 902. ( 1 1 ) Zinato, E.; Adamson, A. W.; Riccieri, P. J . Phys. Chem. 1985, 89, 839. (12) Ramasami, T.; Endicott, J. F.; Brubaker, G. R. J . Phys. Chem. 1983, 87, 5057.
0 1 9 8 7 American Chemical Society
